1. Field of the Invention
The invention concerns opto-electronic systems used to transmit or process optical digital data optically.
2. Description of the Prior Art
Information in such systems is often in the form of binary data represented by pulses modulating an optical carrier wave. The amplitude (or power) of the modulated optical wave determines a binary value.
The signal is inevitably degraded during transmission which makes it more difficult for the receiver to detect the high and low levels of the received signal. With regard to its amplitude, the quality of an optical signal is usually defined by two parameters: the signal to noise ratio and the extinction ratio. The signal to noise ratio is defined as the ratio of the optical power of the signal to the noise power in a band of wavelengths containing the wavelength of the signal carrier. The extinction rate is defined as the ratio of the powers respectively corresponding to the high and low levels of the signal.
The invention concerns an all-optical device, i.e. a device with no optical-to-electrical conversion or vice versa, which improves the quality of a binary optical signal, i.e. increases its extinction rate whilst retaining the highest possible signal to noise ratio. In other words, the device is required to be capable of starting from a poor quality modulated input signal and supplying an output signal with a very high signal to noise ratio and whose high levels are stabilized with a constant optical power and whose low levels are at a very low power.
One solution already proposed to the problem of increasing the extinction rate is to use an interferometer structure, for example one of the Mach-Zehnder type. The structure has two branches carrying two coherent waves originating from a probe wave and coupled to form the output signal. At least one branch includes a medium whose index varies as a function of the optical power that it conveys and an input signal is introduced into that branch. The power variations of the input signal then modulate the index and the two waves can interfere destructively or constructively according to the power of the input signal.
A structure of the above kind does indeed improve the extinction rate but has the drawback that the conditions for destructive and constructive interference are very constraining on the input signal, regarding in particular its wavelength and most of all its power in the high state. As a result its behavior is very sensitive to fluctuations in these parameters.
An improvement to the above device is described in European patent application EP-A-0813097 published Dec. 17, 1997. The proposed device has two stages in cascade. A first stage acts as a peak limiter supplying a modulating signal as a function of the input signal with its high levels stabilized. A second stage is of the interferometer type mentioned above and receives as its input signal the modulating signal from the first stage.
The function of the first stage is therefore to eliminate any fluctuations in the high levels of the modulating signal fed into the interferometer structure. This assures stable behavior of that structure. What is more, if the first stage is a wavelength converter using a semiconductor optical amplifier, the device is independent of the value of or fluctuations in the wavelength and the polarization of the input optical signal.
In practice, the second stage is a Mach-Zehnder interferometer structure, each branch of which includes or consists of a semiconductor optical amplifier. The behavior of the structure can therefore be optimized by adjusting the bias currents of the amplifiers to obtain a maximum extinction rate at the output.
Accordingly, when the interferometer structure is designed to operate in phase opposition, the low levels of the output signal correspond to the high levels of the modulating signal and therefore to the low levels of the input signal. Because the probe wave is of constant power, equalizing the high levels of the modulating signal equalizes the low levels of the output signal. These low levels are accompanied by a low level of noise produced by the amplifier into which the modulating signal is injected.
However, the above solution has limitations in terms of the extinction rate of the output signal and the power dynamic range, i.e. the acceptable fluctuations in the low and high levels of the input signal.
The aim of the invention is to remedy the drawbacks of the above device and to this end the invention consists in a device for formatting an optical input signal in the form of a first optical wave modulated between low and high power levels, the device including:
a first stage for supplying, as a function of the input signal, a modulating optical signal in the form of a second optical wave modulated between low and high power levels, the high levels being stabilized so that they are not very dependent on the fluctuations in the low and high levels of the input signal, and
a second stage including an interferometer structure adapted to receive the modulating signal and to supply an output signal resulting from respectively destructive or constructive interference of first and second coherent waves when the power of the modulating signal is respectively equal to the high and low levels, the structure including first and second guide branches receiving via first coupling means respectively first and second parts of a third optical wave, the branches being respectively provided with first and second semiconductor optical amplifiers, the first amplifier receiving the modulating signal via second coupling means and the first and second amplifiers respectively supplying the first and second coherent waves, in which device the third optical wave is modulated between low and high power levels in phase opposition to the modulation of the modulating signal and the low and high levels of the third optical wave are stabilized so that they are not very dependent on fluctuations in the low and high levels of the input signal.
The extinction rate is improved by modulating the power of the third signal (i.e. the probe signal) in phase opposition to the modulating signal. What is more, equalizing the low levels of the third signal and the high levels of the modulating signal stabilizes the destructive interference so that the low levels of the output signal can be kept at a low value.
There is a problem concerning the transient xe2x80x9cchirpxe2x80x9d phenomenon, i.e. the optical frequency modulation accompanying the variation in the power of the output wave of the interferometer structure.
To characterize this modulation a transient xe2x80x9cchirpxe2x80x9d parameter a is used, and is defined by the equation:
xcex1=2P.(dxcfx86/dt)/(dP/dt)
where P is the power of the modulated wave and xcfx86 is its phase expressed in radians.
A phase opposition interferometer structure modulator has a relatively high positive value of the parameter a. The parameter a should preferably have a null or even negative value for transmission via positive dispersion coefficient fibers, however. This is the case with xe2x80x9cstandardxe2x80x9d fibers for a carrier wavelength around 1.55 xcexcm, for example. The proposed solution has the advantage of reducing the absolute value of the parameter xcex1.
Another aspect to be considered is that of equalizing the high levels of the output signal. If the first stage does not equalize the low levels of the modulating signal (as is the case using a simple wavelength converter employing a semiconductor optical amplifier), the fluctuations in the low levels can lead to fluctuations in the high levels of the output signals.
However, these fluctuations are attenuated if the interferometer structure is subject to conditions such that the index of the branch that receives the modulating signal is not very dependent on the fluctuations in the low levels of the signal. This index is that of the semiconductor optical amplifier of the branch and, as the index of the amplifier is directly related to its gain, this amounts to saying that the gain must not be highly dependent on these fluctuations.
To satisfy this condition a sufficiently low power could be imposed on the high level of the probe wave to maintain the amplifier in its linear region, independently of possible fluctuations in the low levels of the modulating signal. However, this solution is not favorable from the point of view of the extinction rate of the output signal.
Another possibility for avoiding this drawback is for the power of the high levels of the probe wave to be made sufficiently high to render the fluctuations in the low levels of the modulating signal negligible in comparison with the total optical power injected into the amplifier.
Also, in accordance with a supplementary feature of the invention, the high levels of the third optical wave are adjusted to keep the gain of the amplifiers of the interferometer structure saturated.
Other features and advantages of the invention will become apparent in the following description given with reference to the drawings.